The invention relates to a housing for electrical and/or electronic apparatus, which housing comprises means for protecting the apparatus against leakage to outside of information carrying, electrical and/or magnetic fields and electromagnetic radiation generated during operation of the apparatus, as well as against crosstalk between information carrying conductors and/or conducting parts, and against undesired influencing of the apparatus by electrical and/or magnetic fields and electromagnetic radiation of external origin.
Such a housing is known. It serves to accommodate apparatus and operative personnel that may be present. Such a housing is known in various embodiments.
According to the invention the said housing is characterized by
an inner housing portion acting as a Faraday cage,
an outer housing portion acting as Faraday cage entirely encasing the inner housing portion,
which housing portions are mutually insulated electrically,
a first earth conductor electrically connected to the inner housing portion, and
a second earth conductor electrically connected to the outer housing portion,
which earth conductors are situated at a sufficient
mutual distance and are placed at a sufficient depth in the ground such that on the one hand effective earthing of both housing portions is ensured and on the other hand crosstalk between both housing portions is absent and an earthing loop is avoided.
With this construction according to the invention the apparatus accommodated in the inner housing portion is doubly shielded. A further insight underlying the construction according to the invention is that it is of essential importance to avoid an earthing loop, that is, an essentially conducting connection via the ground between both earth conductors. The second earth conductor serves as safety earth with the purpose of carrying away charges which can be built up by EMP (electromagnetic pulses). The first earth conductor can serve to make negligibly small the influence of "TEMPEST". This is a form of electromagnetic influence of apparatus known in the literature.
The above mentioned phenomena, EMP and TEMPEST, relate to the influencing of the apparatus for protection by external sources. Such safety or shielding is of essential importance in guaranteeing proper operation of the apparatus under even the most extreme electromagnetic, electrical and magnetic conditions.
On the other hand, for example telecommunications apparatus, computer apparatus or the like also has to be protected such that leakage of information to outside, and thereby the possibility of listening in by third parties, is excluded.
Use of only one earth connection gives a galvanic coupling between the two systems to be separated as well as possible. For TEMPEST a separate earth therefore always has to be used which is practically entirely separated galvanically from the safety earth. Nor may there exist any noticeable inductive or capacitive coupling between both earth conductors. It is namely so that such a coupling could also result in crosstalk.
It will be apparent from the above that both earth connections have to be arranged in the ground at a certain minimal effective distance from one another.
The housing is preferably characterized by an electrically conducting sleeve protruding on the side of the outer housing portion and through which the first earth conductor extends.
In a particular embodiment the housing displays a second inner housing portion completely encased by the outer housing portion, which inner portion is connected via electrically conducting elements to the outer housing portion and is electrically insulated from the first inner housing portion.
It will be apparent that this second inner housing portion offers a less effective shielding with respect to TEMPEST than the first inner housing portion; it is in any case locally connected galvanically to the outer housing portion for instance by conducting strips, while the first inner housing portion is completely separated galvanically from that outer housing portion and is entirely separately earthed. The second outer housing portion can thus serve for instance to accommodate less sensitive apparatus or apparatus for which the same requirements with regard to confidentiality of the information do not apply. The second inner housing portion can also be accessible from outside by means of a double-walled door. This door has to be embodied such that in closed position it leaves unimpaired the electrical and/or magnetic properties of the housing.
The housing can further display the special feature that all electrical power cables and information carrying electrical cables enter the housing via the second inner housing portion.
In order to secure as well as possible the apparatus present in the first inner housing portion a housing preferably displays the feature that all electrical power cables and information carrying electrical cables between both housing portions are fed through adjoining walls of both housing portions and that at that position a filter is arranged in all the said cables and/or a shield is arranged around all the said cables.
Both inner housing portions can also be connected to one another via a door or hatch to which the same requirements otherwise apply as for the door briefly described above. After the above it will be apparent that this door has to be embodied such that it is double-walled with a complete galvanic separation between two conducting skin plates which at least in the closed position of the door are connected for electrical conduction to the respective walls of the inner housing portions.
A very simple and thereby cheap but nevertheless reliable construction is obtained with a housing which has the special feature that between the outer housing portion and the or each inner housing portion is situated an insulating layer consisting of foam substance such as polyurethane foam.